Stents with drug-containing amphiphilic polymer coating

ABSTRACT

It is provided that a vascular stent having an amphiphilic polymer coating is loaded with a restenosis inhibiting agent which is sparingly soluble in water, whereby delayed release of the agent takes place after implantation of the stent.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 10/466,144 filed on Jan. 5, 2004 which is a national phaseapplication of PCT/GB02/00104 filed on Jan. 11, 2002 which claimspriority to GB0100760.8 filed on Jan. 11, 2001, the teaching of which isincorporated herein by reference in entirety.

The present invention relates to the delivery of drugs from stentscoated with polymer. In particular the invention relates to delivery offor inhibition of restenosis following stent implantation in thetreatment of cardiovascular disease.

A leading cause of mortality within the developed world iscardiovascular disease. Coronary disease is of most concern. Patientshaving such disease usually have narrowing in one or more coronaryarteries. One treatment is coronary stenting, which involves theplacement of a stent at the site of acute artery closure. This type ofsurgery has proved effective in restoring vessel patency and decreasingmyocardial ischemia. However the exposure of currently used metallicstents to flowing blood can result in thrombus formation, smooth musclecell proliferation and acute thrombotic occlusion of the stent.

Non-thrombogenic and anti-thrombogenic coatings for stents have beendeveloped. One type of balloon expandable stent has been coated withpolymers having pendant zwitterionic groups, specificallyphosphorylcholine (PC) groups, generally described in WO-A-93/01221. Aparticularly successful embodiment of those polymers suitable for use onballoon expandable stents has been described in WO-A-98/30615. Thepolymers coated onto the stent have pendant crosslinkable groups whichare subsequently crosslinked by exposure to suitable conditions,generally heat and/or moisture. Specifically a trialkoxysilylalkyl groupreacts with pendant groups of the same type and/or with hydroxyalkylgroups to generate intermolecular crosslinks. The coatings lead toreduced thrombogenicity.

Fischell, T. A. in Circulation (1996) 94: 1494-1495 describes testscarried out on various polymer coated stents. A thinner uniformpolyurethane coating, having a thickness of 23 μM was observed to have abetter performance than a relatively non uniform thicker layer having athickness in the range 75 to 125 μm. The thicker coatings are furtherdescribed by Van der Giessen, W. J. et al., in Circulation:1996:94:1690-1997.

It has been suggested to utilise coatings on stents as reservoirs forpharmaceutically active agents desired for local delivery.

In U.S. Pat. No. 5,380,299 a stent is provided with a coating of athrombolytic compound and optionally an outer layer of ananti-thrombotic compound. The stent may be precoated with a “primer”such as a cellulose ester or nitrate.

Other drug containing stents and stent coatings are described by Topoland Serruys in Circulation (1998) 98:1802-1820.

McNair et al., in Proceedings of the International Symposium onControlled Release Bioactive Materials (1995) 338-339 describe in vitroinvestigations of release of three model drugs, caffeine, dicloxacillinand vitamin B12, from hydrogel polymers having pendant phosphorylcholinegroups. Alteration of the hydrophilic/hydrophobic ratio of the(hydrophilic) phosphorylcholine monomer 2-methacryloyloxyethylphosphorylcholine, (HEMA-PC) and a hydrophobic comonomer andcrosslinking of the polymer allows preparation of polymers having watercontents when swollen in the range 45 to 70 wt %. Crosslinking isachieved by incorporating a reactive monomer3-chloro-2-hydroxypropylmethacrylate. The tests are carried out onmembranes swollen in aqueous drug solutions at 37° C. The release ratesof the model drugs are influenced by the molecular size, solutepartitioning and degree of swelling of the polymer. Dicloxacillin isfound to have a higher half life for release than its molecular sizewould indicate, and the release profile did not appear to be Fickian.

McNair et al, in Medical Device Technology, December 1996, 16-22,describe three series of experiments. In one, polymers formed of HEMA-PCand lauryl methacrylate crosslinked after coating by unspecified meansare cocoated with drugs onto stents. Release rates of dexamethasone fromthe stent, apparently into an aqueous surrounding environment, wasdetermined. Drug release from cast membranes, as model coatings, showedthat the release rate obeyed Fickian diffusion principles, forhydrophilic solutes. In the third series of tests, a non-crosslinkedpolymer coating, free of drug, coated on a stent, had a significantdecrease in platelet adhesion when coated on a stent used in an ex-vivoarteriovenous shunt experiment. The stent coating method was notdescribed in detail.

Stratford et al. in “Novel phosphorylcholine based hydrogelpolymers:developments in medical device coatings” describe polymersformed from 2-methacryloyloxyethyl phosphorylcholine, a higher alkylmethacrylate, hydroxypropylmethacrylate and a methacrylate estercomonomer having a reactive pendant group. These PC polymers wereinvestigated to determine the feasibility of delivering drugs and modeldrugs. Results are shown for caffeine, dicloxacillin, vitamin B12,rhodamine and dipyridamole. The device on which the drug is coated is aguidewire that is, it is not an implant.

In EP-A-0623354, solutions of drug and polymer in a solvent were used tocoat Wiktor type tantalum wire stents expanded on a 3.5 mm angioplastyballoon. The coating weights per stent were in the range 0.6 to 1.5 mg.Coating was either by dipping the stent in the solution, or by sprayingthe stent from an airbrush. In each case coating involved multiplecoating steps. The drug was for delivery to the vessel wall. The drugssuggested as being useful for delivery from stents were glucocorticoids,antiplatelet agents, anticoagulants, antimitotic agents, antioxidants,antimetabolite agents and antiinflammatory agents. The worked examplesall use dexamethasone delivered from a bioabsorbable polymer.

In U.S. Pat. No. 5,900,246 drugs are delivered from a polyurethanecoated substrate such as a stent. The polyurethanes may be modified tocontrol its compatibility with lipophilic or hydrophilic drugs. Suitabledrugs are antithrombotic agents, antiinflammatory agents such assteroids, antioxidants, antiproliferative compounds and vasodilators.Particularly preferred drugs are lipophilic compounds. A polyurethanecoated stent is contacted with a drug in a solvent which swells thepolyurethane, whereby drug is absorbed into the polyurethane. Selectionof a suitable solvent took into account the swellability of thepolyurethane and the solubility of the drug in the solvent. It wasobserved that lipophilic drugs loaded in this way released more slowlyfrom hydrophobic polymer than more hydrophilic drugs, by virtue ofinteraction of the lipophilic drug with hydrophobic polymer.

In EP-A-0923953 coatings for implantable devices, generally stents,comprise an undercoat comprising particulate drug and polymer matrix,and an overlying topcoat which partially covers the undercoat. The topcoat must be discontinuous in situ, in order to allow release of thedrug from the undercoat. Examples of drugs include antiproliferatives,steroidal and non steroidal antiinflammatories, agents that inhibithyperplasia, in particular restenosis, smooth muscle cell inhibitors,growth factor inhibitors and cell adhesion promoters. The workedexamples use heparin and dexamethasone. The polymer of the undercoat is,for example, hydrophobic biostable elastomeric material such assilicones, polyurethanes, ethylene vinyl acetate copolymers, polyolefinelastomers, polyamide elastomers and EPDM rubbers. The top layer issuitably formed of non-porous polymer such as fluorosilicones,polyethylene glycols, polysaccharides and phospholipids. In theexamples, the undercoat comprised silicone polymer, and coating with thepolymer/drug mixture was carried out by spraying a suspension in whichboth drug and polymer were dispersed, followed by curing of the polymer.

In our earlier specification WO-A-0101957, unpublished at the prioritydate hereof, we describe methods for loading drugs into polymer coatedstents. The polymer coating preferably comprised a crosslinked copolymerof an ethylenically unsaturated zwitterionic monomer with a hydrophobiccomonomer. The drug was intended to be delivered into the wall of thevessel in which the stent was implanted and the thickness of the coatingon the stent was adapted so as to provide higher drug dosage on theouter surface of the stent. The drugs were selected fromantiproliferatives, anticoagulants, vasodilators, anti inflammatories,cytotoxic agents and antiangiogenic compounds.

It is well known to those who work in the area of surfactant chemistrythat it is possible to determine critical micelle concentrations by useof hydrophobic probes, which seek out the hydrophobic interior ofmicelles in preference to remaining in an aqueous environment. Pyrene isone such molecule. Moreover, the fluorescence intensities of variousvibronic fine structures in the pyrene molecules' fluorescence spectrumshows strong environmental effects based upon the polarity of thesolvent in which it is present (Kalyanasundaram, K et al.; JACC, 99(7),2039, 1977). The ratio of the intensity of a pair of characteristicbands (1311) is relevant to the environment. A value for I3:I1 of about0.63 is indicative of an aqueous environment whilst a value of about 1is indicative of a hydrophobic environment.

In WO-A-95/03036 it is suggested that stents are coated withantiangiogenic drugs to inhibit tumour invasion. Many of the drugs aresparingly water soluble. The antiangiogenic agent is delivered from apolymeric carrier, such as a bioerodable or biodegradable polymer.

In WO-A-00/56283, polymers having metal chelating activities are said tohave matrix metalloproteinase (MMP) inhibitory activity. The polymersmay be coated onto a stent. It is suggested that MMP's contribute to thedevelopment of atherosclerotic plaques and post angioplasty restenoticplaques. The MMP inhibiting activity of the polymers is believed to beuseful in inhibiting restenosis. The polymers may be coated onto a stentand may have additional pharmaceutically active agents dispersedtherein, some of which may be sparingly soluble in water. Polymershaving matrix metalloproteinase inhibitory (MMPI) activity are capableof chelating divalent metals, and are generally polymers of unsaturatedcarboxylic acids although sulphonated anionic hydrogels may be used. Oneexample of a monomer for forming a sulphonated anionic hydrogel isN,N-dimethyl-N-methacryloyloxyethyl-N-(3-sulphopropyl) ammonium betaine.Other examples of polymers are acrylic acid based polymers modified withC₁₀₋₃₀-alkyl acrylates crosslinked with di- or higher-functionalethylenically unsaturated crosslinking agents. There is no specificsuggestion of how to provide a coating on a stent comprising both MMPIactive polymer and additional therapeutic agent.

In WO-A-99/01118, antioxidants are combined with antineoplastic drugs toimprove their cytotoxicity. One utility of the antineoplasticcombination is in the treatment of vascular disease. The drugcombination may be administered from a controlled release system.

The crosslinkable polymer of 2-methacryloyloxyethyl-2′-trimethylammoniumethylphosphate inner salt and dodecyl methacrylate with scrosslinking monomer, coated onto a stent and cured, has been shown toreduce restenosis following stent delivery for the treatment ofatherosclerotic conditions. In PCT/GB00/02087 mentioned above, we showthat a range of drugs may be loaded onto the polymer coated stents suchthat delivery of the drug into adjacent tissue takes place.

The present invention relates to a stent having a polymer coating, andcomprising a sparingly water soluble drug which may be delivered over anextended period of time from the stents after placement.

A new intravascular stent comprises a metal body having a coatingcomprising polymer and at least 20 μg per stent of a restenosisinhibiting agent in which the restenosis inhibiting agent is a sparinglywater soluble drug and the polymer in the coating is a cross-linkedamphiphilic polymer which, when swollen with water containing pyrene,has hydrophobic domains observable by pyrene fluorescence having anintensity ratio I3:I1 of at least 0.8.

The agent should be present on the external wall of the stent at aconcentration of at least about 0.05 μg/mm², the area being based on thesurface area of metal.

The ratio of I3:I1 is preferably about 1.

The drug should preferably have a water solubility of less than 1 mg/mgat room temperature and a log P, where P is the partition coefficientbetween octanol and water, of at least 1.5, preferably at least 2 ormore.

Preferably, on at least the outer wall of the stent the, coatingcomprises a layer of the said amphiphilic polymer in which the drug isabsorbed. Additionally there may be drug absorbed into polymer in thecoating on the inner wall.

It may be possible to provide a sufficiently high does of drug on thestent in form of absorbed material. However, sometimes it may bedesirable to provide higher doses than may be loaded into theamphiphilic polymer matrix. For instance, it may be undesirable toincrease the level of polymer on the stent so as to be able to support ahigher loading of drug. In a preferred stent, the coating on the outerwall of the stent comprises an inner layer of the said amphiphilicpolymer, and adhered to said inner layer, crystalline drug. Provision ofcrystalline drug may also confer useful release characteristics on thestent. The crystalline material may be controlled of a particle size,for instance, to confer desired release characteristics which complementthe release of absorbed drug from the polymer coating.

In a preferred embodiment of the invention, the coating on at least theouter wall of the stent comprises an inner layer of the said amphiphilicpolymer and the top coat comprising a non-biodegradable, biocompatiblesemipermeable polymer. The semipermeable polymer is selected so as toallow permeation of drug through the top layer when the stent is in anaqueous environment. In such an environment, the semipermeable polymermay, for instance, be swollen, and it is in this form that it shouldallow permeation of the active drug. A topcoat may confer desirablecontrolled release characteristics. Its use of particular value for thepreferred embodiment where coating comprises crystalline drug adhered toan inner layer of amphiphilic polymer. The topcoat in such an embodimenthas several functions. It provides a smooth outer profile, minimisesloss of drug during delivery, provides a biocompatible interface withthe blood vessel after implantation and controls release of drug fromthe stent into the surrounding tissue in use.

A topcoat is preferably substantially free of drug prior to implantationof the stent.

A topcoat is preferably formed of a cross-linked amphiphilic polymer.The coating may be cross-linked or linear. Preferably it is the same asthe first amphiphilic polymer.

In the present invention, an amphiphilic polymer comprises groupsconferring hydrophilicity and groups conferring hydrophobicity.Preferably the groups conferring hydrophilicity comprise zwitterionicgroups.

Preferably the groups conferring hydrophobicity comprise pendanthydrophobic groups selected from C₄₋₂₄-alkyl, -alkenyl and -alkynylgroups any of which may be substituted by one or more fluorine atoms,aryl, C₇₋₂₄ aralkyl, oligo (C₃₋₄ alkoxy) alkyl and siloxane groups.

Most preferably the polymer is formed from ethylenically unsaturatedmonomers including a zwitterionic monomer and a hydrophobic comonomer.For forming a crosslinkable polymer, the ethylenically unsaturatedmonomers preferably include one or more reactive monomer having apendant reactive group(s) capable of forming intermolecular crosslinks.

Preferably the zwitterionic monomer has the general formula I:

YBX  I

wherein

B is a straight or branched alkylene (alkanediyl), alkyleneoxaalkyleneor alkylene oligo-oxaalkylene chain optionally containing one or morefluorine atoms up to and including perfluorinated chains or, if X or Ycontains a terminal carbon atom bonded to B, a valence bond;

X is a zwitterionic group; and

Y is an ethylenically unsaturated polymerisable group selected from

CH₂═C(R)CH₂O—, CH₂═C(R)CH₂OC(O)—, CH₂═C(R)OC(O)—, CH₂═C(R)O—,CH2=C(R)CH₂OC(O)N(R1)-, R2OOCCR═CRC(O)O—, RCH═CHC(O)O—,RCH═C(COOR2)CH2C(O)O—,

wherein:

R is hydrogen or a C₁-C₄ alkyl group;

R¹ is hydrogen or a C₁-C₄ alkyl group or R¹ is —B—X where B and X are asdefined above; and

R² is hydrogen or a C₁₋₄ alkyl group;

A is —O— or —NR¹—;

K is a group —(CH₂)_(p)OC(O)—, —(CH₂)_(p)C(O)O—, —(CH₂)_(p)OC(O)O—,—(CH₂)_(p)NR³—, —(CH₂)_(p)NR3C(O)—, —(CH2)pC(O)NR³—, —(CH2)pNR³C(O)O—,—(CH2)pOC(O)NR³—, —(CH2)pNR³C(O)NR³— (in which the groups R3 are thesame or different), —(CH2)pO—, —(CH2)pSO3—, or, optionally incombination with B, a valence bond

p is from 1 to 12; and

R³ is hydrogen or a C₁-C₄ alkyl group.

In group X, the atom bearing the cationic charge and the atom bearingthe anionic charge are generally separated by 2 to 12 atoms, preferably2 to 8 atoms, more preferably 3 to 6 atoms, generally including at least2 carbon atoms.

Preferably the cationic group in zwitterionic group X is an amine group,preferably a tertiary amine or, more preferably, a quaternary ammoniumgroup. The anionic group in X may be a carboxylate, sulphate,sulphonate, phosphonate, or more preferably, phosphate group. Preferablythe zwitterionic group has a single monovalently charged anionic moietyand a single monovalently charged cationic moiety. A phosphate group ispreferably in the form of a diester.

Preferably, in a pendant group X, the anion is closer to the polymerbackbone than the cation.

Alternatively group X may be a betaine group (ie in which the cation iscloser to the backbone), for instance a sulpho-, carboxy- orphospho-betaine. A betaine group should have no overall charge and ispreferably therefore a carboxy- or sulpho-betaine. If it is aphosphobetaine the phosphate terminal group must be a diester, i.e., beesterified with an alcohol. Such groups may be represented by thegeneral formula II

—X¹—R⁴—N⁺(R⁵)₂—R⁶—V  II

in which X¹ is a valence bond, —S— or —NH—, preferably —O—;

V is a carboxylate, sulphonate or phosphate (diester-monovalentlycharged) anion;

R⁴ is a valence bond (together with X¹) or alkylene —C(O)alkylene- or—C(O)NHalkylene preferably alkylene and preferably containing from 1 to6 carbon atoms in the alkylene chain;

the groups R⁵ are the same or different and each is hydrogen or alkyl of1 to 4 carbon atoms or the groups R⁵ together with the nitrogen to whichthey are attached form a heterocyclic ring of 5 to 7 atoms; and

R⁶ is alkylene of 1 to 20, preferably 1 to 10, more preferably 1 to 6carbon atoms.

One preferred sulphobetaine monomer has the formula II

where the groups R⁷ are the same or different and each is hydrogen orC₁₋₄ alkyl and d is from 2 to 4.

Preferably the groups R⁷ are the same. It is also preferable that atleast one of the groups R⁷ is methyl, and more preferable that thegroups R⁷ are both methyl.

Preferably d is 2 or 3, more preferably 3.

Alternatively the group X may be an amino acid moiety in which the alphacarbon atom (to which an amine group and the carboxylic acid group areattached) is joined through a linker group to the backbone of polymer A.Such groups may be represented by the general formula IV

in which X² is a valence bond, —O—, —S— or —NH—, preferably —O—,

R⁹ is a valence bond (optionally together with X²) or alkylene,—C(O)alkylene- or —C(O)NHalkylene, preferably alkylene and preferablycontaining from 1 to 6 carbon atoms; and

the groups R⁸ are the same or different and each is hydrogen or alkyl of1 to 4 carbon atoms, preferably methyl, or two of the groups R⁸,together with the nitrogen to which they are attached, form aheterocyclic ring of from 5 to 7 atoms, or the three group R⁸ togetherwith the nitrogen atom to which they are attached form a fused ringstructure containing from 5 to 7 atoms in each ring.

X is preferably of formula V

in which the moieties X³ and X⁴, which are the same or different, are—O—, —S—, —NH— or a valence bond, preferably —O—, and W⁺ is a groupcomprising an ammonium, phosphonium or sulphonium cationic group and agroup linking the anionic and cationic moieties which is preferably aC₁₋₁₂-alkanediyl group.

Preferably W contains as cationic group an ammonium group, morepreferably a quaternary ammonium group.

The group W⁺ may for example be a group of formula —W¹—N⁺R¹⁰ ₃—W¹—P⁺R¹¹₃—W¹—S⁺R¹¹ ₂ or —W¹-Het⁺ in which:

W¹ is alkanediyl of 1 or more, preferably 2-6 carbon atoms optionallycontaining one or more ethylenically unsaturated double or triple bonds,disubstituted-aryl, alkylene aryl, aryl alkylene, or alkylene arylalkylene, disubstituted cycloalkyl, alkylene cycloalkyl, cycloalkylalkylene or alkylene cycloalkyl alkylene, which group W¹ optionallycontains one or more fluorine substituents and/or one or more functionalgroups; and

either the groups R¹⁰ are the same or different and each is hydrogen oralkyl of 1 to 4 carbon atoms, preferably methyl, or aryl, such as phenylor two of the groups R¹⁰ together with the nitrogen atom to which theyare attached form a heterocyclic ring containing from 5 to 7 atoms orthe three groups R¹⁰ together with the nitrogen atom to which they areattached form a fused ring structure containing from 5 to 7 atoms ineach ring, and optionally one or more of the groups R¹⁰ is substitutedby a hydrophilic functional group, and

the groups R¹¹ are the same or different and each is R¹¹ or a groupOR¹¹, where R¹¹ is as defined above; or

Het is an aromatic nitrogen-, phosphorus- or sulphur-, preferablynitrogen-, containing ring, for example pyridine.

Preferably W¹ is a straight-chain alkanediyl group, most preferablyethane-1,2 diyl.

Preferred groups X of the formula V are groups of formula VI:

where the groups R¹² are the same or different and each is hydrogen orC₁₋₄ alkyl, and e is from 1 to 4.

Preferably the groups R¹² are the same. It is also preferable that atleast one of the groups R¹² is methyl, and more preferable that thegroups R¹² are all methyl.

Preferably e is 2 or 3, more preferably 2.

Alternatively the ammonium phosphate ester group VIII may be replaced bya glycerol derivative of the formula VB, VC or VD defined in our earlierpublication No. WO-A-93/01221.

Preferably the hydrophobic comonomer has the general formula VII

Y¹R¹³  VII

wherein Y¹ is selected from

CH₂═C(R¹⁴)CH₂O—, CH₂═C(R¹⁴)CH₂OC(O)—, CH₂═C(R¹⁴)OC(O)—, CH₂═C(R¹⁴)O—,CH₂═C(R¹⁴)CH₂OC(O)N(R¹⁵)—, R¹⁶OCCR¹⁴═CR¹⁴C(O)O—, R¹⁴CH═CHC(O)O—,R¹⁴CH═C(COOR¹⁶)CH₂C(O)—O—,

wherein:

R¹⁴ is hydrogen or a C₁-C₄ alkyl group;

R¹⁵ is hydrogen or a C₁-C₄ alkyl group or R¹⁵ is R¹³;

R¹⁶ is hydrogen or a C₁₋₄ alkyl group;

A¹ is —O— or —NR¹⁵—; and

K¹ is a group —(CH₂)_(q)OC(O)—, —(CH₂)_(q)C(O)O—, (CH₂)_(q)OC(O)O—,—(CH₂)_(q)NR¹⁷—, —(CH₂)_(q)NR¹⁷C(O)—, —(CH₂)_(q)C(O)NR¹⁷—,(CH₂)_(q)NR¹⁷C(O)O—, —(CH₂)_(q)OC(O)NR¹⁷—, —(CH₂)_(q)NR¹⁷C(O)NR¹⁷— (inwhich the groups R¹⁷ are the same or different), —(CH₂)_(q)O—,—(CH₂)_(q)SO₃—, or a valence bond;

p is from 1 to 12;

and R¹⁷ is hydrogen or a C₁-C₄ alkyl group;

and R¹³ is the hydrophobic group.

In the comonomer of the general formula VII, the group R¹³ is preferablya hydrophobic group, preferably:

a) a straight or branched alkyl, alkoxyalkyl or oligoalkoxyalkyl chaincontaining 4 or more, preferably 6 to 24 carbon atoms, unsubstituted orsubstituted by one or more fluorine atoms optionally containing one ormore carbon double or triple bonds;

b) an aryl or aralkyl, preferably phenyl, phenethyl or benzyl; or

c) a siloxane group —(CR¹⁸ ₂)_(qq)(SiR¹⁹ ₂)(OSiR¹⁹ ₂)_(pp)R¹⁹ in whicheach group R¹⁸ is the same or different and is hydrogen or alkyl of 1 to4 carbon atoms, or aralkyl, for example benzyl or phenethyl, each groupR¹⁹ is alkyl of 1 to 4 carbon atoms, qq is from 1 to 6 and pp is from 0to 49

Most preferably R¹³ is a straight alkyl having 4 to 18, preferably 12 to16 carbon atoms.

The reactive monomer to which provides crosslinkability preferably hasthe general formula VIII

Y²B²R²⁰  VIII

wherein

B² is a straight or branched alkylene, oxaalkylene or oligo-oxaalkylenechain optionally containing one or more fluorine atoms up to andincluding perfluorinated chains, or B² is a valence bond;

Y² is an ethylenically unsaturated polymerisable group selected from

CH₂═C(R²¹)CH₂—O—, CH₂═C(R²¹)CH₂OC(O)—, CH₂═C(R²¹)OC(O)—, CH₂═C(R²¹)O—,CH₂═C(R²¹)CH₂OC(O)N(R²²)—, R²³OOCCR²¹═CR²¹C(O)O—, R²¹H═CHC(O)O—,R²¹H═C(COOR²³)CH₂C(O)O—

where

R²¹ is hydrogen or C₁-C₄ alkyl;

R²³ is hydrogen, or a C₁₋₄-alkyl group;

A² is —O— or —NR²²—;

R²² is hydrogen or a C₁-C₄ alkyl group or R²² is a group B²R²⁰;

K² is a group —(CH₂)_(k)OC(O)—, —(CH)_(k)C(O)O—, —(CH₂)_(k)OC(O)O—,—(CH₂)_(k)NR²²—, —CH₂)_(k)NR²²C(O)—, —(CH₂)_(k)OC(O)O—, —CH₂)_(k)NR²²—,—(CH₂)_(k)NR²²C(O)—, —(CH₂)_(k)C(O)NR²²—, —(CH₂)_(k)NR²²C(O)O—,—(CH₂)_(k)OC(O)NR²²—, —(CH₂)_(k)NR²²C(O)NR²²— (in which the groups R²²are the same or different), —(CH₂)_(k)O—, —(CH₂)_(k)SO₃—, a valence bondand k is from 1 to 12; and

R²⁰ is a cross-linkable group.

Group R²⁰ is selected so as to be reactive with itself or with afunctional group in the polymer (eg in group R¹³) or at a surface to becoated. The group R²⁰ is preferably a reactive group selected from thegroup consisting of ethylenically and acetylenically unsaturated groupcontaining radicals; aldehyde groups; silane and siloxane groupscontaining one or more substituents selected from halogen atoms andC₁₋₄-alkoxy groups; hydroxyl; amino; carboxyl; epoxy; —CHOHCH₂Hal (inwhich Hal is selected from chlorine, bromine and iodine atoms);succinimido; tosylate; triflate; imidazole carbonyl amino; optionallysubstituted triazine groups; acetoxy; mesylate; carbonyl di(cyclo)alkylcarbodiimidoyl; isocyanate, acetoacetoxy; and oximino. Most preferablyR²⁰ comprises a silane group containing at least one, preferably threesubstituents selected from halogen atoms and C₁₋₄-alkoxy groups,preferably containing three methoxy groups.

Preferably each of the groups Y to Y² is represented by the same type ofgroup, most preferably each being an acrylic type group, of the formulaH₂C═C(R)C(O)-A, H₂C═C(R¹⁴)C(O)A¹ or H₂C═C(R²¹)C(O)-A2, respectively.Preferably the groups R, R¹⁴ and R²¹ are all the same and are preferablyH or, more preferably, CH₃. Preferably A, A¹ and A² are the same and aremost preferably —O—. B and B² are preferably straight chainC₂₋₆-alkanediyl.

Preferably the ethylenically unsaturated comonomers comprise diluentcomonomers which may be used to give the polymer desired physical andmechanical properties. Particular examples of diluent comonomers includealkyl(alk)acrylate preferably containing 1 to 24 carbon atoms in thealkyl group of the ester moiety, such as methyl (alk)acrylate or dodecylmethacrylate; a dialkylamino alkyl(alk)acrylate, preferably containing 1to 4 carbon atoms in each alkyl moiety of the amine and 1 to 4 carbonatoms in the alkylene chain, e.g. 2-(dimethylamino)ethyl (alk)acrylate;an alkyl (alk)acrylamide preferably containing 1 to 4 carbon atoms inthe alkyl group of the amide moiety; a hydroxyalkyl (alk)acrylatepreferably containing from 1 to 4 carbon atoms in the hydroxyalkylmoiety, e.g. a 2-hydroxyethyl (alk)acrylate glycerylmonomethacrylate orpolyethyleneglycol monomethacrylate; or a vinyl monomer such as anN-vinyl lactam, preferably containing from 5 to 7 atoms in the lactamring, for instance vinyl pyrrolidone; styrene or a styrene derivativewhich for example is substituted on the phenyl ring by one or more alkylgroups containing from 1 to 6, preferably 1 to 4, carbon atoms, and/orby one or more halogen, such as fluorine atoms, e.g.(pentafluorophenyl)styrene.

Other suitable diluent comonomers include polyhydroxyl, for examplesugar, (alk)acrylates and (alk)acrylamides in which the alkyl groupcontains is from 1 to 4 carbon atoms, e.g. sugar acrylates,methacrylates, ethacrylates, acrylamides, methacrylamides andethacrylamides. Suitable sugars include glucose and sorbitol. Diluentcomonomers include methacryloyl glucose and sorbitol methacrylate.

Further diluents which may be mentioned specifically includepolymerisable alkenes, preferably of 2-4 carbon atoms, eg. ethylene,dienes such as butadiene, ethylenically unsaturated dibasic acidanhydrides such as maleic anhydride and cyano-substituted alkenes, suchas acrylonitrile.

Particularly preferred diluent monomers are nonionic monomers, mostpreferably alkyl(alk)acrylates or hydroxyalkyl(alk)acrylates.

It is particularly desirable to include hydroxyalkyl(alk)acrylates incombination with reactive comonomers which contain reactive silylmoieties including one or more halogen or alkoxy substituent. Thehydroxyalkyl group containing monomer may be considered a reactivemonomer although it also acts as a diluent. Such reactive silyl groupsare reactive with hydroxy groups to provide crosslinking of the polymerafter coating, for instance.

A particularly preferred combination of reactive monomers isω(trialkoxysilyl)alkyl(meth)acrylate and anω-hydroxyalkyl(meth)acrylate.

The monomers may, in some embodiments, comprise an ionic comonomer.Suitable comonomers are disclosed in our earlier publicationWO-A-9301221.

Preferably the zwitterionic monomer is used in the monomer mixture in amolar proportion of at least 1%, preferably less than 75%, morepreferably in the range 5 to 50%, most preferably 10-33%. Thehydrophobic comonomer is generally used in molar proportion of at least2%, preferably at least 5% or at least 10%, more preferably in the range15 to 99%, especially 50 to 95%, more especially 60 to 90%. Thecross-linkable monomer is preferably used in a molar amount in the range2 to 33%, preferably 3 to 20%, more preferably 5 to 10% by mole.

The zwitterionic polymer can be represented by the general formula IX:

in which I is 1 to 75, m is 0 to 99, n is 0 to 33 and m+n is 25 to 99,Y³to Y⁵ are the groups derived from Y to Y², respectively, of the radicalinitiated addition polymerisation of the ethylenic group in Y to Y², and

B and X are as defined for the general formula I,

R¹³ is as defined for the general formula VII, and

B² and R²⁰ are as defined for the general formula VIII.

In the preferred zwitterionic polymer in which Y, Y¹ and Y² are eachacrylic groups the polymer has the general formula X

in which B, X, R and A are as defined for the compound of the generalformula I, R¹⁴, A¹ and R¹³ are as defined for the general formula VII,R²¹, D², B² and R²⁰ are as defined for the general formula VII and I, mand n are as defined for the general formula IX

The polymerisation is carried out using suitable conditions as known inthe art. Thus the polymerisation involves radical initiation, usingthermal or redox initiators which generate free radicals and/or actinic(e.g. u.v or gamma) radiation, optionally in combination withphotoinitiators and/or catalysts. The initiator is preferably used in anamount in the range 0.05 to 5% by weight based on the weight of monomerpreferably an amount in the range 0.1 to 3%, most preferably in therange 0.5 to 2%. The level of initiator is generally higher where themonomer includes reactive monomer and the polymer is cross-linkable,e.g. 1 to 20%.

The molecular weight of the polymer (as coated, where the polymer iscross-linkable) is in the range 1×10⁴ to 10⁶, preferably in the range5×10⁴ to 5×10⁵ D.

The monomer mixture and the monomer mixture may include anon-polymerisable diluent, for instance a polymerisation solvent. Such asolvent may provide solubility and miscibility of the monomers. Thesolvent may be aqueous or non-aqueous. The polymer may be recovered byprecipitation from the polymerisation mixture using a precipitatingsolvent, or recovery may involve removal of any non polymerisablediluent by evaporation, for instance.

In the present invention the term sparingly water soluble means that atroom temperature the solubility of the compound in water is less than 1ml. The restenosis inhibiting agent (drug) is preferably a compoundhaving a log P, where P is the octanol/water partition coefficient, ofat least 1.5 for instance more than 2.

The drug should have a log P of at least 1.5, preferably at least 2.When such drugs are absorbed into polymers having the hydrophobicdomains, observed using the pyrene fluorescence test, interactionapparently takes place, affecting the release rate of the drug. It isbelieved that the hydrophobic drug will preferentially partition intothe hydrophobic domains in a similar fashion to pyrene.

Preferred drugs are steroids, especially estrogens, especially estradiol(E2) and corticosteroids, especially dexamethasone (Dex) and6α-methylprednisolone (MP). Estradiol has a log P of 4.3 and a watersolubility of 0.003 mg/ml; MP has a log P of 1.42; Dex has a log P of2.55 and a water solubility of 0.01 mg/ml. Other examples of sparinglywater-soluble drugs which may be used in the invention include statins,such as simvastatin (log P 2.06), and, though less preferably lovastatin(log P 1.7) and atorvastatin (log P 1.6).

Preferably the drug should be present in an amount in the range 20 to1000 preferably at least 50 μg, more preferably in the range 100 to 400μg, per stent.

The stent may be made of a shape memory metal, or may be elasticallyself-expanding, for instance, be a braided stent. However, preferably itis a balloon expandable stent. In the embodiment of the invention, inwhich a topcoat is provided, the topcoat may be part of a coherentcoating formed over both a stent and a stent delivery device, forinstance a balloon of a balloon catheter from which a balloon expandablestent is delivered. In this case, the balloon may additionally beprovided with a coating comprising drug, for instance adsorbed ontoparts of its exterior surface between stent struts. Such a device may beproduced by loading the stent with drug after the stent has been mountedonto the delivery catheter.

According to a further aspect of the invention there is provided a newmethod for producing a drug coated intravascular stent comprising thesteps:

a) a metallic stent body is coated on its inner and outer walls with across-linkable amphiphilic polymer;

b) the cross-linkable polymer is subjected to conditions under whichcross-linking takes place to produce a stent coated with polymer which,when swollen with water containing pyrene, has hydrophobic domainsobservable by pyrene fluorescence having an intensity ratio I3:I1 of atleast 0.8.;

c) at least the outer coated wall of the polymer coated stent iscontacted with liquid drug composition comprising a sparinglywater-soluble restenosis inhibiting drug and an organic solvent in whichthe drug is at least partially dissolved and which is capable ofswelling the cross-linked polymer of the coating, for a time sufficientto swell the polymer coating on the outer wall, to produce a wetdrug-coated stent in which drug is present in an amount of at least 20μg per stent;

d) organic solvent is evaporated from the wet stent to produce a drydrug-coated stent.

In the method of the invention, in step c), the drug may be bothabsorbed into the polymer and adsorbed at the process of the polymercoating whereby, upon evaporation of the solvent in step d) crystals ofdrug are formed which are adherent to the surface of the dry drug coatedstent.

In the method of the invention, contact of the polymer coated stent withthe liquid drug composition may be by dipping the stent into a body ofthe stent, and/or by flowing, spraying or dripping liquid compositiononto the stent with immediate evaporation of solvent from the wet stent.Such steps allow good control of drug loading onto the stent, and areparticularly useful for forming the crystals of drug at the surface ofpolymer.

Whilst the stent may be provided with drug coating in the inventionprior to being mounted onto its delivery device, it is preferred, andmost convenient, for the stent to be premounted onto its delivery deviceprior to carrying out step c). By this means, it is primarily the outerwall of the stent (as opposed to the inner wall of the stent) whichbecomes coated with drug. Whilst this method will generally result indrug being coated onto the stent delivery section of the deliverycatheter, this is, in general, not disadvantageous. In somecircumstances it may be useful for the outer surface of the deliverycatheter to be provided with a coating of drug, which may be deliveredto adjacent tissue upon placement of the stent in use. Generally thedelivery catheter is in contact with such tissue for a short period,whereby contact is not maintained for a prolonged period, and limitedlevel of transfer of drug from the balloon take place.

The method of the invention may include a step of applying a topcoat. Insuch a method a further step e) is carried out:

e) to at least the outer wall of the dry drug coated stent a polymer isapplied, to form a non-biodegradable biocompatible semi-permeablepolymer-containing top-coat.

In this preferred embodiment, in step e) it is preferred that a liquidtop-coating composition comprising polymer is coated onto at least theouter wall and is cured after coating to form the top-coat. It isdesirable for the liquid coating to be sprayed onto the outer wall ofthe stent, as this method has been found to minimise removal ofpreviously applied drug.

The top-coating composition, and consequently the top coat in theproduct, should generally be substantially free of drug. Preferably itis substantially free of other pharmaceutical actives although incertain circumstances it may be useful to cocoat a mixture of polymerand another pharmaceutically active agent.

For the embodiment of the invention where the liquid top-coatingcomposition comprises a cross-linkable polymer of the type preferred foruse to form the first amphiphilic polymer, the liquid top-coatingcomposition comprises cross-linkable polymer and the curing step in thepreferred method involves exposure of the top-coat to cross-linkingconditions.

Curing of cross-linkable polymer may involve exposure to irradiation,chemical curing agents, catalysts or, more usually raised temperatureand/or reduced pressure to acceptable condensation based cross-linkingreactions. Drying the liquid during composition usually involves raisedtemperature and/or reduced pressure for a time sufficient to reduce theamount of solvent remaining on the stent to undetectable levels orlevels at which it will not interfere with subsequent processing steps,or with release of the drug in use, or be toxic to a patient in whom thestent is implanted.

Where in the preferred method, the stent is preloaded onto its deliverydevice before being coated with drug, the top-coat is provided over boththe stent and the stent delivery section of the delivery catheter.Preferably the top-coat forms a coherent film covering the entire stentdelivery section. It is preferred for the device subsequently to besterilised and to be packaged into a sterile package for storage priorto use. Sterilisation may involve y irradiation, or application of heat,but preferably involves contact with ethylene oxide.

Where, in the preferred method, a stent is contacted with liquid drugcomposition whilst mounted on a delivery device, it is important toensure that the said contact does not adversely effect the properties ofthe delivery catheter. For a balloon catheter, the contact must notsignificantly reduce the burst strength of the balloon. A preferredballoon catheter used for delivering a stent is formed of polyamide. Wehave established that the use of ethanol, methanol or dimethylsulfoxide(DMSO) do not damage the balloon such that burst strength are reduced toan unacceptable level. The examples of solvents and solvent mixturesinclude dichloromethane (DCM), mixtures of isopropanol and water andDCM/ethanol mixtures.

The solvent must be selected to allow adequate dissolution of drug, andswelling of the cross-linked polymer coating to allow absorption of druginto the body of the polymer. Drug which is absorbed into the polymerwill be released over a period of time after implantation of the stent.The liquid drug composition may comprise other components, such ascrystal modifiers, polymers, salts, acids, bases etc. It may beconvenient to include dissolved amphiphilic, optionally crosslinkablepolymer, to confer compatibility with the polymer on the stent surface.Such a polymer may be identical to that described above used in thefirst aspect of the invention.

According to a further aspect of the invention there is also provided anintravascular stent comprising a metal body and a coating on the metalbody comprising an amphiphilic polymer and 17β-estradiol. Preferably theamphiphilic polymer has a hydrophobic domain observable by pyrenefluorescence having an intensity ratio I3:I1 of at least 0.8, preferablyabout 1.0. Preferably at least 20 μg estradiol is incorporated perstent.

According to a further aspect of the invention there is also provided anintravascular stent comprising a metal body and a coating on the metalbody comprising an amphiphilic polymer and 6α-methylprednisolone.Preferably the amphiphilic polymer has a hydrophobic domain observableby pyrene fluorescence having an intensity ratio I3:I1 of at least 0.8,preferably about 1.0. Preferably at least 20 μg MP is incorporated perstent. Preferred embodiments of these further aspects of the inventionare preferably also covered by the main aspects of the invention andcomprise the preferred aspects thereof and are produced by the inventivemethod.

The present inventors have established that stents according to theinvention confer improved quantitative coronary angioplasty results whenused in animals, reduced intimal hyperplasia, increased lumen diameterand excellent clinical results as compared to control polymer coatedstents.

The drawings relate to the following:

FIG. 1 compares the I3:I1 ratio from the fluorescence spectra of pyrenein different environments (Ref Ex. 1);

FIG. 2 shows the amount of pyrene retained in a variety of polymercoatings as determined in Reference Example 1;

FIG. 3A compares the fluorescence spectra of pyrene in laurylmethacrylate and water;

FIG. 3B compares the fluorescence spectra of pyrene in water and in twoamphiphilic polymers (Ref Ex 1)

FIGS. 4 a and 4 b show the actual and theoretical release rates ofdexamethasone and 17β-estradiol from polymers (Ref Ex 2)

FIG. 5 shows the estradiol elution profile for Example 2;

FIG. 6 shows the estradiol elution profile for Example 3;

FIG. 7 shows the estradiol elution profile for Example 4; and

FIG. 8 shows the 6α-methyl prednisolone elution profile for Example 7.

The present invention is illustrated in the accompanying examples:

REFERENCE EXAMPLE 1

Zwitterionic polymer coatings were investigated by allowing pyrene todiffuse into the polymer and studying the degree to which it is takenup, and the effects on the ratio of the fluorescence band intensities tosee if there is any significant indication of the type of environmentpresent.

Polymer coatings of interest were dissolved in an appropriate solvent(usually ethanol) at 20 mgml⁻¹. The solution was used to coatpolymethylmethacrylate (PMMA) fluorescence cuvettes by simply pouringinto the cuvette, draining, following by an oven curing at 70° C.overnight. Polymers studied were:

a) a copolymer of 2-methacryloyloxy ethyl-2′-trimethyl-ammoniumethylphosphate inner salt (MPC): n-butylmethacrylate: hydroxypropylmethacrylate (HPM): trimethoxysilylpropylmethacylate (TSM) 29:51:15:5(by weight)

b) a copolymer of MPC: benzylacrylate: HPM:TSM 29:51:15:5

c) a copolymer of MPC: dodecylmethacrylate (DM): HPM: TSM: 45:35:15:5

d) a copolymer of MPC: DM: HPM: TSM: 29:51:15:5

e) a copolymer of MPC: DM: HPM: TSM: 15:65:15:5

f) poly(2-hydroxyethylmethacrylate).

The copolymers a-e were synthesised as disclosed in WO-A-9830615.

Analytical grade pyrene was used in high purity water (8.32×10⁻⁴ M). Thefluorescence spectrum was recorded using an excitation wavelength of 335nm and scanned from 350-440 nm on a PE LS 50B LuminescenceSpectrophotometer. Subtraction of the spectrum of each coating in waterwas necessary to remove the interference of a small band at 380 nmpresent in all methacrylate systems.

Environment information could be obtained by comparing the ratio of theintensity of the peaks at 373 nm (I1) and 383 nm (I3) (I3/I1). WhereI3/I1 was similar for polymer systems, the comparative amount of pyrenepresent could be estimated by the maximum intensity of I1;alternatively, the entire peak area may offer an alternative measure ofthe comparative amount of pyrene present in different coatings. It wasimportant to mark the side of the cuvette to ensure the sameorientations were achieved each time it was replaced in thespectrophotometer.

FIG. 3A compares the fluorescence spectra of pyrene in laurylmethacrylate (dodecyl methacrylate) (8.32×10⁻⁴ M) and water (8.32×10⁻⁵M). For water the I3/I1 ratio is 0.633 (literature value¹ 0.63) and theI3/I1 ratio for lauryl methacrylate is 1.029. This indicates the verydifferent environments than might be expected to be seen within thepolymer coating.

Pyrene solution added to the coated cuvettes was allowed to stand for 16h, the cuvette emptied and washed throughly with ultrapure water,refilled with ultrapure water and the fluorescence spectrum recorded.The comparative maximum height of I1 was used to estimate the relativeamounts of pyrene in the coatings. This was repeated for three cuvettesof each polymer and the average taken. Despite some variations betweencuvettes, the trends were the same, indicating that the polymerformulations with more hydrophobic content seemed to contain more pyrene(FIG. 2). This is in contradiction to the water contents of thesematerials which vary in the opposite order. Hence for the varyingsystems, although water contents vary in the order c>d>f>e(88:40:38:27), the final fluorescence intensity (loading of pyreneachieved in the coating) varies according to e>d>c≧f. This indicatesthat the pyrene is preferentially associating itself with hydrophobicareas within the coating.

The ratio of I3/I1 was also studied (FIG. 1) and again, those polymerwith formal hydrophobic chains showed a greater ratio (indicating morehydrophobic environment for the pyrene). This polymer containing thebenzyl side chain has a lower than expected I3/I1, initially indicatingpoor interaction with the pyrene. However, measurement of the I3/I1 forpyrene in the pure benzyl acrylate monomer showed that the maximum I3/I1that could be expected would be 0.75 (i.e. less of a shift influorescent intensity is produced in this aromatic monomer compared tothe lauryl monomer). PHEMA coating showed I3/I1 characteristic of pyrenein an aqueous environment (FIG. 3A), suggesting no formal hydrophobicdomain exists.

REFERENCE EXAMPLE 2 Drug-Polymer Interaction Versus Drug Solubility

There are examples of stent-based release of therapeutics that rely uponthe poor solubility of the active agent in water to achieve a slowrelease rate, i.e. by relying for extended release of drug on poorsolubility of the drug in water. When a graph of solubility versusrelease time (T90%) is plotted however, the relationship is extremelypoor (R²=0.006) indicating the solubility on its own does not accountfor the observed release characteristics.

This can be modelled further by comparing the theoretical release ofdrug into a known volume of water based purely upon its solubility andcomparing this with its actual release profile from the polymer systeminto the same elution volume. Assuming that 100 μg of the drug is placeon a surface, and that the drug is eluted off into 5 ml of solution, andthen at various arbitrary points, 1 ml removed, and 1 ml of freshsolution added, the dissolution profiles for various drugs could becalculated and compared to experimental data obtained the same way. Thevariation between calculated and observed could be attributed to theinteraction with the polymer matrix.

This is clearly illustrated by FIGS. 4 a and b. Here, the theoreticalrelease of dexamethasone (which has a log P where P is the partitioncoefficient between octanol and water of 2.55) and estradiol have beencalculated and plotted on the graph (diamonds/circles) based on thesolubility of the compound and the volume of water into which it isbeing eluted (Details of further loading and elution studies frompolymer-coated stents are given below). The difference between this lineand the observed data (squares) is the degree of interaction of thecompound with the hydrophobic domains within the polymer coating whichin this case is polymer d) from Ref. Example 1. It is the interactionthat prolongs the release of the compound and offers some capability tocontrol the delivery of the drug to its surrounding environment.

EXAMPLE 1 Estradiol Uptake Studies

1.1 Normal (Low) Loading Level

15 mm BiodivYsio DD stents provided with a cross-linked coating on bothinner and outer walls of copolymer d) used in Reference Example 1 wereprovided with a coating of drug by immersing them in a 20 mg/ml solutionof estradiol for 30 minutes, removing the stents from the solution andwick drying them on tissue then allowing them to dry for 2 hours at roomtemperature. The drug total loading was measured by HPLC, and found tobe in the range 45-65 μg per stent.

1.2 High Loading Level

18 mm BiodivYsio stents premounted on their balloon delivery catheterwere coated by dipping the balloon and stent in a volume of a 20 mg/mlsolution of estradiol in ethanol for 5 minutes, removing and drying for5 minutes, then pipetting 10 μl of the drug solution on to thestent/balloon and allowing to dry for 1 minute, this pipetting anddrying step being repeated once, with a final 10 minute drying period.The balloons were inflated, deflated and the stents removed. The levelof drug on each stent and balloon was determined using HPLC. The levelof drug on the stents was in the range 225-250 μg per stent, and theamount on each balloon was about 190-220 μg.

1.3 High Loading Level Repeat

The method of example 1.2 was repeated with the stents premounted oneither a 3 mm or 4 mm diameter balloon. For the 4 mm balloon system themean level of drug was 240 with a range for 3 samples of 229-254 μg,giving an area distribution of 2.4 μg/mm² (range 2.3 to 2.6). For the 3mm balloon system the mean loading per stent was 2.59 μg with a rangefor 5 samples of 243-276 μg. The area distribution is 2.6 μg/mm² (range2.4-2.8).

EXAMPLE 2 Estradiol Elution Studies at 25° C.-Non-Flow System

Elution studies were carried out at 25° C. for upto 1 hour in gentlyagitated PBS. This was done by placing 15 mm DD stents loaded withestradiol, from Example 1.1 individually in vials containing 5 mlphosphate buffered saline (PBS) on rollers. At various time intervals upto 7 hours a 1 ml aliquot was removed and replaced with 1 ml fresh PBS.The stents and water aliquots were measured, to give the amount of drugeluted and the amount of drug remaining on the stents. The results areshown in FIG. 5. It may be seen that during the first hour estradiol waseluted relatively more quickly than the rest of the time and at 4 hoursthere was still 62% estradiol still remaining on the stent.

EXAMPLE 3 Release of 17β-Estradiol

The elution profile for 17β-estradiol, FIG. 6 from the stents producedin example 1.1, was assessed using an in-vitro test where five stentswere vigorously stirred in a large volume (1000 ml) of PBS salinesolution at 37° C. This test demonstrates that the stent is capable of asustained release of 17β-estradiol over a duration of at least severalhours. Aliquots of buffer were removed at various time points over a 24hour period and analysed for 17β-estradiol content. The results areshown in FIG. 6. When translated into in-vivo conditions, the releaseprofile is predicted to be over a prolonged period of time.

EXAMPLE 4 Estradiol Elution Studies in Flow-System

The elution of estradiol was examined in a flow system at 37° C. andevaluated over an 8 hour period. PBS was maintained at 37° C. in sixstirred reservoirs (500 ml each) within a water bath. A length ofsilicone tubing (3 mm internal diameter) was attached from eachreservoir to one of six stent chambers (4 mm internal diameter 80 mmlong) and back to the respective reservoir via a peristaltic pump. Thesystem was pumped using a flow rate of 100 ml/min to reach equilibriumtemperature of 37° C. The flow was stopped and two estradiol loaded 15mm stents loaded as in Example 1 were placed in each of the six stentchambers, and flow recommenced. A stent was then removed at various timeperiods and wick dried. These were used to measure the amount ofestradiol remaining on the stent. The results are shown in FIG. 7. Thisshows that in this model the estradiol was eluted relatively morequickly than in the stirred 5 ml PBS of Example 2. Since the totalvolume of PBS passing over the stents in the flow model is 500 ml, it islikely that throughout the period the rate of desorption of drug fromthe stent was higher than the rate of absorption from the environment.This condition may not apply to the non-flow method.

EXAMPLE 5 In Vivo Test on Estradiol Loaded Stents

This study investigated the acute and short term effects of deployingestradiol (17β) loaded 18 mm stents produced generally as described inExample 1 above (ie loaded with drug whilst mounted on the balloon usingeither a single dipping step or the multi-step loading method) inporcine arteries. There were 3 arms to the study: 1) Control, using thenon-drug-coated 18 mm BiodivYsio stent with the polymer coating d fromthe reference example 1, 2) low estradiol dose (about 45-65 μg perstent) using the dip only loading method of Example 1.1, and 3) highdose (about 225-250 μg per stent) by using the multi-step loading method(Ex. 1.2).

A total of 6 animals were each implanted with three stents, one each ofthe control, low and high dose, one stent in each of three coronaryarteries. A balloon:artery ratio of about 1.25:1 (in the range(1.2-1.3):1) was used, the oversizing designed to cause an injury to theartery wall resulting in neointimal formation resembling that occurringin stented human coronary arteries.

One month after implantation observations were made by quantitativecoronary angiogram (QCA) of the mean lumen diameter (MLD). Subsequentlythe results were evaluated by histomorphometric analysis of intimalhyperplasia formation and vessel luminareas, as well as for the extentof re-endothelialisation. The results are shown in Table 1.

TABLE 1 MLD mm Intimal Area mm² (S.D.) Luminal area mm² (S.D) Control2.26 4.31 (1.1) 3.49 (1.41) Low 2.31  3.60 (0.79) 4.20 (1.74) High 2.532.54 (1.0) 5.40 (1.70)

The study showed a 40% reduction in intimal area in the ‘High Dose’17β-estradiol loaded stents compared with control stents (p<0.05), seeFIG. 4. There was also a reduction in the Intimal Area/injury scoreratio in the ‘High Dose’ 17β-estradiol group compared with the ‘Control’stents (1.32±0.40 mm² vs 1.96.±−0.32 mm², for 17β-estradiol vs controlrespectively, P<0.01). There was no significant difference in the injuryscore for all three study arms.

A trend was noted for the Luminal Area where there was an increase inLuminal Area with an increase in dosage.

Re-endothelialization scores were high for all three study arms,suggesting that 17b-estradiol does not inhibit the healing process.

EXAMPLE 6α Methylprednisolone Uptake Studies

6.1 Low Loading Level

15 mm BiodivYsio DD stents provided with a cross-linked coating on isboth inner and outer walls of copolymer d) used in Reference Example 1were provided with a coating of drug by immersing them in a 12.35 mg/mlsolution of 6α-methyl prednisolone (MP) for 5 minutes, removing thestents from the solution and wick drying them on tissue then allowingthem to dry for at least 1 hour at room temperature. The drug totalloading was measured by placing the stent in ethanol (9.0 ml) andsonicated for 30 minutes. The concentration in the ethanol wasdetermined by UV at 246.9 nm compared to standards. The loading wasfound to be in the range 30-40 μg per stent.

6.2 High Loading Level

18 mm BiodivYsio stents coated with the cross-linked polymer d) on bothwalls premounted on their balloon delivery catheter were coated bydipping the balloon and stent in a volume of a 12.0 mg/ml solution of MPin ethanol for 5 minutes, removing and drying for 5 minutes, thenpipetting 10 μl of the drug solution on to the stent/balloon andallowing to dry for 1 minute, this pipetting and drying step beingrepeated thrice, with a final 10 minute drying period. The balloons wereinflated, deflated and the stents removed. The level of drug on eachstent was determined using the technique described above. The level ofdrug on the stents was in the range 250-300 μg per stent.

EXAMPLE 7 MP Elution Studies at 25° C.-Non Flow System

Elution studies were carried out at 25° C. for up to 1 hour in gentlyagitated PBS. This was done by placing 15 mm DD stents loaded with MPfrom Examples 6.1 and 6.2 individually in vials containing 5 mlphosphate buffered saline (PBS) on rollers. At various time intervals a1 ml aliquot was removed and replaced with 1 ml fresh PBS. The stentsand water aliquots were measured, to give the amount of drug eluted andthe amount of drug remaining on the stents. The results are shown inFIG. 8.

EXAMPLE 6 In Vivo Test on MP and Dexamethasone Loaded Stents

This study investigated the acute and short term effects of deploying MPand dexamethasone loaded 18 mm stents produced generally as described inExample 1 above (i.e. loaded with drug whilst mounted on the balloonusing either the single dipping step of example 1.1) or the multi-steploading method of example 1.2) in porcine arteries. There were 4 arms tothe study: 1) Control, using the non-drug-coated 18 mm BiodivYsio DDstent (which is coated with polymer d), 2) low Dex dose (about 95.mu.gper stent) using the dip only loading method, 3) high Dex dose (about265 μg per stent) by using the multi-step loading method, and 4) highdose MP produced as described in Example 6.2 (about 270 μg per stent).

Stents were implanted into porcine arteries for 5 days, explanted, thenassessed for inflammation by H&E staining and the results scoredhistopathologically and morphometrically on an arbitrary scale.

From nine measurements for each data point the following results wereobtained: (p values in parentheses in table), 1=no difference, 0.05=95%confidence of a difference), see table 2.

TABLE 2 Histopathological findings Inflammation Injury ThrombusPerivaculitis Control 0.73 0.56 0.74 0.48 +Dex Low dose 0.73 0.53 0.770.51 +Dex High dose 0.57 0.40 0.54* 0.45 +MP 0.51* 0.42 0.50* 0.39Morphometric Result Dia Area IEL Dia- EEL Dia- Sten (%) Sten (%) Lum.Dia Lum. Dia Control 5 9 0.16 0.47 +Dex Low dose 4 (0.02) 8 (0.14)  0.14(0.18) 0.47 (1) +Dex High dose 4 (0.02) 7 (0.005) 0.13 (0.009) 0.43(0.24) +MP 3 (0) 6 (0.001) 0.11 (00 0.35 (0) *indicates statisticallysignificant difference from control (p = 0.05)

The results of Examples 6 and 8 show that the high dose stents show atrend towards improved results.

EXAMPLE 9 Re-Endothelialisation of Dexamethasone=Loaded Stents

The dexamethasone-loaded stents (Low Dex) described in example 8 wereimplanted into porcine coronary arteries for 30 days. After this timethe animals were sacrificed and the stented sections of the arteriesremoved and fixed. The vessel was cut longitudinally and opened out toexpose the inner surface which was sputter coated and viewed under bySEM. SEM revealed that the inner surface of the vessel had completelyre-endothelialised over the stent struts.

EXAMPLE 10 Clinical Trial Assessment—30 Day Data for 71 Patients

Study of anti-restenosis with the BiodivYsio Dexamethasone eluting stent(STRIDE) which is a multi-centre prospective study performed at 7centres in Belgium with 71 patients. The primary objective of this studywas to evaluate the proportion of patients with binary restenosis 6months after receiving a BiodivYsio stent loaded with dexamethasone i.e.produced by the same technique as the LowDex stent described in example8. The secondary objectives were to evaluate the incidence of sub(acute)thrombosis to 30 days post procedure and the occurrence of MACE (death,recurrent myocardial infarction or clinically driven target lesionrevascularisation) at 30 days and 6 months post procedure.

11, 15, 18 and 28 mm by 3.0 to 4.0 mm diameter BiodivYsio stents loadedwith dexamethasone were under investigation. 30 day data for 71 patients(safety analysis set) are reported in this example. Other endpoints havenot yet been reached and therefore will not be described.

71 patients (79% male) with an average height of 170 cm and weight of 79Kg were enrolled into the study. 63% of patients had a history ofhypercholesterolaemia and 69% had smoked or were current smokers. 47% ofpatients had multi-vessel disease and 44% had a history of previous MI.The vessels/lesions treated were in the following categories:

Vessel Treated Lesion Classification RCA 31% A 21% LAD 41% B1 48% Cx 19%B2 27% Other  9% C  4%

The mean lesion length treated was 9 mm. The majority of patients hadeither a 15 mm (34%) or an 18 mm (39%) stent implanted.

At 30 day follow-up two patients had a MACE (1 patient died one day postprocedure following coronary embolism and 1 patient had a non Q-wave MI)(Table 3). Three patients had serious adverse events that were unrelatedto the study treatment

Technical device success defined as intended stent successfullyimplanted as the first stent was 95%. Clinical device success defined astechnical device success in the absence of MACE to discharge wasachieved in 94% of patients.

The data presented in this initial interim analysis suggest that thepresence of dexamethasone in the coating is not associated with anincreased occurrence of MACE or serious adverse events and that theBiodivYsio Dexamethasone stent is safe in the short term for use inpatients.

REFERENCE EXAMPLE 3 Assessment of Changing Solvent on DD Stent DeliverySystem

In order to load the pre-mounted (on a balloon delivery catheter(balloon formed from a nylon blend)) DD stent with non-water solubledrug, the stent/delivery system combination must be immersed in the drugsolution. The aim of this experiment was to check if the solvent had adetrimental effect on the balloons. Pre-mounted BioDivYsio stents wereplaced in solvent for minutes then allowed to air dry to 5 minutes. Themechanical properties of the balloon were then assessed by a burstpressure test.

The samples were connected to a pressure pump and gauge and a positivepressure of 1 atm (10⁵ Pa) applied and left for 30 seconds. The pressurewas increased by 1 atm (10⁵ Pa) every 30 seconds until the stent wasfully deployed i.e. there were no creases or folds in the balloon.

The pressure was then increased to 16 atm which is the rated burstpressure for the balloon system, and held for 30 seconds. The pressurewas then increased in 1 atm. steps and held for 30 seconds at each step,until the balloon burst. The results are in Table 2.

TABLE 3 Effect of Drug Loading Solvent on Balloon Burst PressureDeployment Burst Solvent Pressure/atm. Pressure/atm. None 3 >16 Ethanol3 >16 Methanol 3 23 ± 1 DMSO 3 24 ± 1

None of the solvents cause detrimental effects on the balloon. Thechoice of drug loading solvent is therefore related to drying rate andsolvent toxicity, drug solubility, and swellability of the polymer.

1. An intravascular stent comprising a metal body and a coating comprising a polymer and at least 20 μg per stent of a restenosis inhibiting agent in which the restenosis inhibiting agent is a sparingly water soluble drug and the polymer in the coating is a cross-linked amphiphilic polymer which, when swollen with water containing pyrene, has hydrophobic domains observable by pyrene fluorescence having an intensity ratio I3:I1 of at least 0.8, wherein the amphiphilic polymer comprises, as the groups conferring hydrophobicity, pendant hydrophobic groups selected from C₄₋₂₄-alkenyl and -alkynyl groups any of which is optionally substituted by one or more fluorine atoms, aryl, C₇₋₂₄ aralkyl, oligo (C₃₋₄ alkoxy) alkyl and siloxane groups, as the groups conferring hydrophilicity, zwitterionic groups.
 2. The stent according to claim 1, wherein on at least the outer wall of the stent the coating comprises a layer of the said amphiphilic polymer in which the drug is absorbed.
 3. The stent according to claim 1, wherein the polymer in the coating when swollen with water containing pyrene has hydrophobic domains observable by I3:I1 ratio of about
 1. 4. The stent according to claim 1, wherein the C₄₋₂₄-alkenyl and -alkynyl groups are substituted by one or more fluorine atoms, aryl, C₇₋₂₄ aralkyl, oligo (C₃₋₄ alkoxy) alkyl and siloxane groups.
 5. The stent according to claim 1, wherein the polymer is formed from ethylenically unsaturated monomers including a zwitterionic monomer and a hydrophobic monomer having said hydrophobic group.
 6. The stent according to claim 5, wherein the ethylenically unsaturated monomers include one or more reactive monomers having a pendant reactive group capable of forming intermolecular crosslinks.
 7. The stent according to claim 5, wherein the zwitterionic monomer has the general formula I: YBX  I wherein B is a straight or branched alkylene (alkanediyl), alkyleneoxaalkylene or alkylene oligo-oxaalkylene chain optionally containing one or more fluorine atoms up to and including perfluorinated chains or, if X or Y contains a terminal carbon atom bonded to B, a valence bond; X is a zwitterionic group; and Y is an ethylenically unsaturated polymerisable group selected from

CH₂═C(R)CH₂O—, CH₂═C(R)CH₂OC(O)—, CH₂═C(R)OC(O)—, —CH₂═C(R)O—, CH₂═C(R)CH₂OC(O)N(R¹)—, R²OOCCR═CRC(O)O—, RCH═CHC(O)O—, RCH═C(COOR²)CH₂C(O)O—,

wherein: R is hydrogen or a C₁-C₄ alkyl group; R¹ is hydrogen or a C₁-C₄ alkyl group or R¹ is —B—X where B and X are as defined above; and R² is hydrogen or a C₁₋₄ alkyl group; A is —O— or —NR¹—; K is a group —(CH₂)_(p)OC(O)—, —(CH₂)_(p)C(O)O—, —(CH₂)_(p)OC(O)O—, —(CH₂)_(p)NR³—, —(CH₂)_(p)NR³C(O)—, —CH₂)_(p)C(O)NR³—, —(CH₂)_(p)NR³C(O)O—, —(CH₂)_(p)OC(O)NR³—, —(CH₂)_(p)NR³C(O)NR³— (in which the groups R³ are the same or different), —(CH₂)_(p)O—, —(CH₂)_(p)SO₃—, or, optionally in combination with B, a valence bond, p is from 1 to 12; and R³ is hydrogen or a C₁-C₄ alkyl group.
 8. The stent according to claim 7, wherein the cationic group in X is an amine.
 9. The stent according to claim 8, wherein the cationic group is a quaternary ammonium group.
 10. The stent according to claim 7, wherein the anionic group in X is selected from sulphate, sulphonate, phosphate, phosphonate and carboxylate.
 11. The stent according to claim 10, wherein X is selected from groups of the general formula V:

in which the moieties X³ and X⁴, which are the same or different, are —O—, —S—, —NH— or a valence bond, and W⁺ is a group comprising an ammonium, phosphonium or sulphonium cationic group and a group linking the anionic and cationic moieties.
 12. The stent according to claim 5, wherein the hydrophobic monomer has the general formula VII Y¹R¹³  VII wherein Y¹ is selected from

CH₂═C(R¹⁴)CH₂O—, CH₂═C(R¹⁴)CH₂OC(O)—, CH₂═C(R¹⁴)OC(O)—, CH₂═C(R¹⁴)O—, CH₂═C(R¹⁴)CH₂OC(O)N(R¹⁵)—, R¹⁶OOCCR¹⁴═CR¹⁴C(O)O—, R¹⁴—CH═CHC(O)O—, R¹⁴—CH═C(COOR¹⁶)CH₂C(O)—O—,

wherein: R¹⁴ is hydrogen or a C₁-C₄ alkyl group; R¹⁵ is hydrogen or a C₁-C₄ alkyl group or R¹⁵ is R¹³; R¹⁶ is hydrogen or a C₁₋₄ alkyl group; A¹ is —O— or —NR¹⁵—; K¹ is a group —(CH₂)_(q)OC(O)—, —(CH₂)_(q)C(O)O—, (CH₂)_(q)OC(O)O—, —(CH₂)_(q)(NR¹⁷—, —(CH₂)_(q)NR¹⁷C(O)—, —(CH₂)_(q)C(O)NR¹⁷—, —(CH₂)_(q)NR¹⁷C(O)O—, —(CH₂)_(q)OC(O)NR¹⁷—, —(CH₂)_(q)NR¹⁷C(O)NR¹⁷— (in which the groups R¹⁷ are the same or different), —(CH₂)_(q)O—, —(CH₂)_(q)SO₃—, or a valence bond, p is from 1 to 12; R¹⁷ is hydrogen or a C₁-C₄ alkyl group; and R¹³ is the hydrophobic group.
 13. The stent according to claim 12, wherein R¹³ is selected from a) C₄₋₁₆ alkyl groups; b) aryl and aralkyl; and c) a siloxane group —(CR¹⁸ ₂)_(qq)(SiR¹⁹ ₂)(OSiR¹⁹ ₂)_(pp)R¹⁹ in which each group R¹⁸ is the same or different and is hydrogen or alkyl of 1 to 4 carbon atoms, or aralkyl, for example benzyl or phenethyl, each group R¹⁹ is alkyl of 1 to 4 carbon atoms, qq is from 1 to 6 and pp is from 0 to
 49. 14. The stent according to claim 6, wherein each reactive monomer has the general formula VIII Y²B²R²⁰  VIII wherein B² is a straight or branched alkylene, oxaalkylene or oligo-oxaalkylene chain optionally containing one or more fluorine atoms up to and including perfluorinated chains, or B² is a valence bond; Y² is an ethylenically unsaturated polymerisable group selected from

CH₂═C(R²¹)CH₂—O—, CH₂═C(R²¹)CH₂OC(O)—, CH₂═C(R²¹)OC(O)—, CH₂═C(R²¹)O—, CH₂═C(R²¹)CH₂OC(O)N(R²²)—, R²³OOCCR²¹═CR²¹C(O)O—, R²¹H═CHC(O)O—, R²¹H═C(COOR²³)CH₂C(O)O—

where R²¹ is hydrogen or C₁-C₄ alkyl; R²³ is hydrogen, or a C₁₋₄-alkyl group; A² is —O— or —NR²²—; R²² is hydrogen or a C₁-C₄ alkyl group or R²² is a group B²R²⁰; K² is a group —(CH₂)_(k)OC(O)—, (CH)_(k)C(O)O—, —(CH₂)_(k)OC(O)O—, —(CH₂)_(k)NR²²—, —(CH₂)_(k)NR²²C(O)—, —(CH₂)_(k)OC(O)O—, —(CH₂)_(k)NR²²—, —(CH₂)_(k)NR²²C(O)—, —(CH₂)_(k)C(O)NR²²—, —(CH₂)_(k)NR²²C(O)O—, —CH₂)_(k)OC(O)NR²²—, —(CH₂)_(k)NR²²C(O)NR²²— (in which the groups R²² are the same or different), —(CH₂)_(k)O—, —(CH₂)_(k)SO₃—, a valence bond and k is from 1 to 12; and R²⁰ is a cross-linkable group.
 15. The stent according to claim 14, wherein R²⁰ is selected from the group consisting of ethylenically and acetylenically unsaturated group containing radicals; aldehyde groups; silane and siloxane groups containing one or more substituents selected from halogen atoms and C₁₋₄-alkoxy group, hydroxyl, amino, carboxyl, epoxy, —CHOHCH₂Hal (in which Hal is selected from chlorine, bromine and iodine atoms), succinimido, tosylate, triflate, imidazole carbonyl amino, optionally substituted triazine groups, acetoxy; mesylate, carbonyl di(cyclo)alkyl carbodiimidoyl, isocyanate, acetoacetoxy, and aximino.
 16. The stent according to claim 14, wherein R²⁰ comprises a silane group containing at least one, preferably three substituents selected from halogen atoms and C₁₋₄-alkoxy groups, more preferably containing three methoxy groups.
 17. The stent according to claim 1, wherein the restenosis inhibiting agent has a log P where P is the partition coefficient between octanol and water in the range 1.2-4.5.
 18. The stent according to claim 1, wherein the restenosis inhibiting agent is a steroid, or a corticosteroid.
 19. The stent according to claim 1, wherein the restenosis inhibiting agent is present in an amount in the range of 20 to 1000 μg per stent. 